LATER TRIASSIC DELTAS IN SHANXI-GANSU-NINGXIA BASIN

Yanchang series of the Triassic in Shanxi-Gansu-Ningxia Basin is lacustrine facies consisting of developed source rocks, reservoirs and cap rocks. Its depositing center is in the south of the basin where multiple-stage deltas were formed. They are the target stratum inoil exploration of low permeability sandstones. Wuqi Delta is a typical one in these deltas. It has a perfect vertical sequence made up by the bottom, front and top of the delta. If the distribution of the delta is mapped with drilling data within 6000 km2, it looks just like a lobate. It is one of the construction-type deltas, which were formed under the condition of parity energy from two directions of lake and river.Through delineating the deltas of different priods and connecting well by well,it may be seen that it was formed by overlapping four deltas of different periods and extended toward the lake center. The drilling results are described in details, and sand bodies both in the distributary streams of the delta plain and in the delta front are regarded as predominant reservoir belts, thus providing the basis for estimating this delta. Having described Wuqi Delta, the author analysed and summarized the development characteristics of the Later Triassic deltas in Shanxi-Gansu -Ningxia Basin and found out the following preliminary rules: ( 1 ) The deltas existing in a basin are always in groups, not in individuals; ( 2 ) The developing scale of deltas is mainly controlled by the supply degree of deposits; ( 3 ) There is a direct propotion between the vertical sequence thickness of the delta and its plane distribution area. The thicker the vertical sequence, the thicker the main sandbody and the better the permeability and porosity as well as the prospecting result; ( 4 ) Along with the formation, development and disappearance of the basin, the deltaic area has undergone a developing process from small to large and then to small. The period of deposit peak was slightly later than the heyday of basin formation; (5) There were major constuction periods and certain development areas for a delta in the basin. After each construction period, regional pla-inazation or swampezation occurred, thus a favourable prospecting area matched with source rocks, reservoirs and cap rocks was formed; ( 6 ) There are two kinds of combination types of deltas in various periods, overlap type and spread type. The former is characterized by overlapping of multiple-stage deltas and distributed in the northeast of the basin, which is the most favourable area to exploration. The later trans-versally spreads in wide area and is distributed in the western margin of the basin, where there would be a large oil area once oil is found there; ( 7 ) Oil prospects of deltaic system throughout the northeast part of this basin, the match relationship of source rocks and reservoir rocks, trap conditions and exploration difficulties are also discussed and evaluated. Finally, the author makes some suggestions concerning further drilling work.